1. Field of the Invention
The present invention relates to an optical fiber which can be suitably used as an optical transmission path and a dispersion compensator.
2. Related Background Art
FIG. 22 is a cross-sectional view of an optical fiber including a so-called microstructure which has been known conventionally. As shown in FIG. 22, this optical fiber has a cross-sectional structure having a large number of voids (vacant holes) 62 in a silica glass 61. A central portion in cross section having no voids 62 constitutes a core region 63 and a portion, surrounding the core region 63, which has a large number of the voids 62 constitutes a cladding region 64.
The principle of light confinement of the optical fiber having such a microstructure is explained qualitatively using a concept called effective refractive indices (for example, T. A. Birks et al. Optics Letters Vol. 22 p.961 (1997)). Due to the existence of the microstructure, in a strict sense, the refractive indices in the core region 63 and the cladding region 64 should have a complicate distribution. However, on the assumption that the optical guide characteristics can be approximated by replacing respective regions with uniform mediums, the refractive indices of these uniform mediums are called the effective refractive indices. The effective refractive indices neff satisfy a following equation.                                           (                                                            f                  1                                                  n                  1                  2                                            +                                                f                  2                                                  n                  2                  2                                                      )                                -            1                          ≤                  n          eff          2                ≤                                            f              1                        ⁢                          n              1              2                                +                                    f              2                        ⁢                          n              2              2                                                          (        1        )            where n is the refractive index and f is the volume fraction. Further, a suffix 1 indicates silica glass and a suffix 2 indicates air. With respect to the volume refraction, f1+f2 =1 is established. Usually, since n1>n2, the both side members of the equation (1) become smaller corresponding to the increase of f2. Accordingly, the effective refractive index of the cladding region 64 having a large number of voids 62 becomes smaller than the effective refractive index of the core region 63 so that the light confinement is realized in the same manner as in the usual optical fiber.
Such a model of the effective refractive indices is considered to be reasonable in a case that the optical wavelength is large compared to the scale of the microstructure. However, as the optical wavelength becomes shorter, the light is locally concentrated at portions having the high refractive index and hence, although the effective refractive indices are elevated, simultaneously, it is considered that the assumption that the structure having refractive index distribution can be replaced by the uniform mediums will lose the validity.
On the other hand, an optical fiber having a greater negative chromatic dispersion than such an optical fiber is disclosed in U.S. Pat. No. 5,802,236, for example. Although this optical fiber has the above-mentioned microstructure, the optical fiber is characterized in that a cladding region is constituted by an inner cladding region and an outer cladding region and the effective refractive index of the inner cladding region is smaller than the outer cladding region.